The invention is directed to water-soluble pressure-sensitive adhesives and dissolvable pressure-sensitive adhesive labels.
Containers used in the restaurant industry typically have a label attached to indicate the content, expiration date and other necessary information. Such containers are reuseable after being cleaned. However, for reuse of the container, it is desirable that the label, including the adhesive used to attach the label to the container, be cleanly removed. Adhesive residue left on the container after washing can lead to bacteria growth. Normally such containers are passed through an industrial dishwasher, which washes at approximately 60xc2x0 C. for about 30 seconds. With commercially available labels, this process frequently leaves behind adhesive residue or results in a ghosting effect, i.e., an image of the label that is stained onto the container, most likely due to migration of the adhesive into the container and the cohesive failure effect of the adhesive. The ghosting effect occurs most frequently on plastic containers. A need exists for an adhesive that can be used for such applications so that, under such washing conditions, there is no adhesive residue or ghosting effect. Additionally, the adhesive and facestock need to dissolve or break up into very small particles so that they can be eliminated with the water and not clog the dishwasher.
The present invention provides dissolvable adhesives and dissolvable adhesive label constructions that address the above concerns. The dissolvable pressure-sensitive adhesive (PSA) comprises an emulsion acrylic copolymer. The emulsion acrylic copolymer is formed from a plurality of monomers comprising at least one alkyl (meth)acrylate, at least one N-vinyl lactam monomer, and at least one hydroxy (meth)acrylate ester. In a preferred embodiment, the plurality of monomers further comprises at least one modifying monomer. In another preferred embodiment, the copolymer is blended with a post-additive to enhance the dissolvability of the PSA. Preferably the copolymer has a weight average molecular weight less than about 120,000.
In a particularly preferred embodiment, the plurality of monomers comprises at least one N-vinyl lactam monomer present in an amount ranging from about 1% to about 15% by weight, at least one hydroxy (meth)acrylate ester present in an amount ranging from about 1% to about 15% by weight, at least one alkyl (meth)acrylate present in an amount ranging from about 35% to about 80% by weight, based on the total weight of the monomers; and at least one ethylenically unsaturated carboxylic acid monomer present in an amount ranging from about 4% to about 15% by weight, based on the total weight of the monomers.
In an even more particularly preferred embodiment, the plurality of monomers comprises at least one N-vinyl lactam monomer present in an amount ranging from about 4% to about 12% by weight, at least one hydroxy (meth)acrylate ester present in an amount ranging from about 1% to about 9% by weight, at least one alkyl (meth)acrylate present in an amount ranging from about 50% to about 70% by weight, at least one ethylenically unsaturated carboxylic acid monomer present in an amount ranging from about 6% to about 12% by weight, at least one modifying monomer present in an amount ranging from about 0.05% to about 3% by weight, and at least one vinyl ester present in an amount ranging from about 11% to about 15% by weight, based on the total weight of the monomers.
In another preferred embodiment, the adhesive, when coated on a dissolvable acetate facestock and subjected to a dissolvability test (as described herein), dissolves in less than 40 seconds.
The present invention is directed to dissolvable pressure-sensitive adhesives (PSAs) and dissolvable PSA constructions utilizing the dissolvable PSAs. The PSA comprises an emulsion acrylic copolymer formed from a plurality of monomers comprising at least one alkyl (meth)acrylate, at least one N-vinyl lactam monomer and at least one hydroxy (meth)acrylate ester.
Examples of suitable N-vinyl lactam monomers for use in the present invention include N-vinyl-2-pyrrolidone, 5-methyl-N-vinyl-2-pyrrolidone, 5-ethyl-N-vinyl-2-pyrrolidone, 3,3-dimethyl-N-vinyl-2-pyrrolidone, 3-methyl-N-vinyl-2-pyrrolidone, 3-ethyl-N-vinyl-2-pyrrolidone, 4-methyl-N-vinyl-2-pyrrolidone, 4-ethyl-N-vinyl-2-pyrrolidone, 1-vinyl-2-piperidone, N-vinyl-2-valerolactam, N-vinyl-2-caprolactam, and mixtures thereof. The N-vinyl lactam monomer (or mixture of N-vinyl lactam monomers) is preferably present in the monomer mixture in a total amount ranging from about 1% to about 15% by weight, more preferably from about 4% to about 12% by weight, even more preferably from about 6% to about 9% by weight, based on the total weight of the monomer mixture.
Examples of suitable hydroxy (meth)acrylate esters for use in the present invention include hydroxymethyl acrylate, 4-hydroxybutyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, hydroxymethyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, and hydroxybutyl methacrylate. The hydroxy (meth)acrylate ester (or mixture of hydroxy (meth)acrylate esters) is preferably present in the monomer mixture in a total amount ranging from about 1% to about 15% by weight, more preferably from about 1% to about 9% by weight, still more preferably from about 3% to about 9% by weight, based on the total weight of the monomer mixture.
In a particularly preferred embodiment, the monomer mixture further includes a modifying monomer to increase the shear of the adhesive. The modifying monomer is selected from nitrogen-containing heterocyclic methacrylates and internal crosslinkers. An example of a suitable nitrogen-containing heterocyclic methacrylate for use in the present invention is 1-2-methacryloxyethyl)imidazolidin-2-one (commercially available in a 50/50 mix with methyl acrylate under the name Norsocryl(copyright) 104, from Elf Atochem North America, Inc., Philadelphia, Pa.). Examples of suitable internal crosslinkers include multifunctional acrylates and methacrylates, such as diacrylates (ethylene glycol diacrylate, propylene glycol diacrylate, polyethylene glycol diacrylate, and hexanediol diacrylate), dimethacrylates (ethylene glycol diacrylate, diethylene glycol dimethacrylate, and 1,3 butane glycol dimethacrylate), triacrylates (trimethylolpropane trimethacrylate, ethoxylated trimethylolpropane triacrylate, and pentaerythritol triacrylate), and trimethacrylates (pentaerythritol trimethacrylate and trimethylolpropane trimethacrylate), as well as divinyl esters, such as divinylbenzene, divinyl succinate, divinyl adipate, divinyl maleate, divinyl oxalate, divinyl malonate, and divinyl glutarate. The modifying monomer (or mixture of modifying monomers) is preferably present in the monomer mixture in a total amount ranging from about 0.02% to about 4% by weight, more preferably from about 0.05% to about 3% by weight, based on the total weight of the monomers. If Norsocryl(copyright) 104 is used, it is preferably present in an amount ranging from about 0.1% to about 0.5% by weight, based on the total weight of the monomers.
The monomer mixture further comprises other suitable monomers typically used in the preparation of PSAs, such as alkyl (meth)acrylates, carboxylic acid monomers, vinyl esters and styrenic monomers. Examples of suitable alkyl (meth)acrylates for use in the present invention include methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, isobornyl acrylate, isononyl acrylate, isodecyl acrylate, methylbutyl acrylate, 4-methyl-2-pentyl acrylate, butyl methacrylate, 2-ethylhexyl methacrylate, isooctyl methacrylate and mixtures thereof. The one or more alkyl (meth)acrylates are preferably present in the monomer mixture in a total amount ranging from about 35% to about 80% by weight, more preferably from about 50% to about 70% by weight, based on the total weight of the monomers.
Examples of suitable ethylenically unsaturated carboxylic acids for use in the present invention include acrylic acid, methacrylic acid, itaconic acid, fumaric acid and mixtures thereof. The one or more ethylenically unsaturated carboxylic acids are preferably present in the monomer mixture in a total amount ranging from about 4% to about 15% by weight, more preferably from about 6% to about 12% by weight, based on the total weight of the monomers.
Examples of suitable vinyl esters useful in the present invention include vinyl acetate, vinyl butyrate, vinyl propionate, vinyl isobutyrate, vinyl valerate, vinyl versatate and mixtures thereof. The one or more vinyl esters are preferably present in the monomer mixture in a total amount ranging from about 8% to about 18% by weight, more preferably from about 11% to about 15% by weight, based on the total weight of the monomers.
Examples of suitable styrenic monomers include styrene and alpha methyl styrene.
The dissolvable emulsion acrylic copolymers of the present invention can be prepared by any suitable technique, preferably by free-radical polymerization in the presence of suitable polymerization initiators and emulsifiers (surfactants). The polymerization is preferably conducted in an oxygen-free atmosphere. One or more activators, redox agents and chain transfer agents are also preferably employed.
Enough polymerization initiator is used to promote free-radical polymerization of the monomers. Preferably the polymerization initiators are water-soluble initiators. Non-limiting examples of suitable water-soluble polymerization initiators include persulfates, such as sodium and potassium persulfate; peroxides, such as hydrogen peroxide and tert-butyl hydroperoxide; and azo compounds, such as VAZO initiators, used alone in combination with one or more reducing agents or activators, for example, bisulfites, metabisulfites, ascorbic acid, erythorbic acid, sodium formaldehyde sulfoxylate, ferrous sulfate, ferrous ammonium sulfate, and ferric ethylenediamine tetraacetic acid.
Non-ionic, anionic, and/or cationic surfactants are advantageously used in the emulsion polymerization of acrylic monomers, with a plurality of surfactants being preferred. Enough surfactant is included to form a stable emulsion without causing phase separation. Nonlimiting examples of surfactants useful in the practice of the present invention include Alipal CO-436, a sodium nonylphenol ethoxylate sulfate surfactant (available from Rhone-Poulenc, Cranbury, N.J.), AR-150, a nonionic, ethoxylated rosin acid emulsifier (available from Hercules, Inc., Wilmington, Del.), Trem LF-40, a sodium alkyl allyl sulfosuccinate surfactant (available from Henkel of America, Inc., King of Prussia, Pennsylvania), sodium pyrophosphate (available from J. T. Baker, Mallickrodt Baker, Inc., Phillibsburg, N.J.), Disponsil FES77, a sodium lauryl ether surfactant (available from Henkel of America, Inc.), Aerosol OT-75, a sodium dioctyl sulfosuccinate surfactant (available from American Cyanamid, Wayne, N.J.), Polystep B-22, a sodium nonylphenol ethyoxylated sulfate (available from Stepan Company, Inc., Winnetka, Ill.), and J927, a mixture of sodium nonylphenol ethoxylate sulfate and nonylphenol ethoxylate (available from Stepan Company). Preferably a sodium nonylphenol ethoxylate sulfate surfactant is included to increase the dissolvability of the resulting PSA copolymer. The sodium nonylphenol ethoxylate sulfate surfactant is preferably provided in the monomer mixture in a total amount ranging from about 4% to about 12% by weight, more preferably from about 6% to about 10% by weight, based on the total weight of the plurality of monomers in the monomer mixture.
It is also preferred to employ a small amount (e.g., from about 0.01 to 0.5% by weight based on the total weight of the monomers) of a chain transfer agent or other molecular weight regulator to control average polymer chain length of the acrylic copolymers. Nonlimiting examples include n-dodecyl mercaptan (n-DDM), t-dodecyl mercaptan (t-DDM), monothioglycerol, mercapto acetates, and long chain alcohols.
The emulsion copolymers of the invention are prepared with excellent conversions at a reaction temperature of around 70xc2x0 C., in the presence of a persulfate or equivalent catalyst in an amount of from about 0.5% to about 1% by weight based on the total weight of the monomers. The monomer mixture is preferably fed over a period of about 3 hours. The reaction pH can be adjusted to within a range of from about 4.0 to about 6.0 by addition of sodium bicarbonate or a similar agent.
While actual production techniques may vary depending upon the particular monomer composition employed, available equipment and other considerations, in general the emulsion polymers are prepared by first mixing one or more pre-emulsions containing conventional surfactants, sodium bicarbonate, and some or all of the monomers in deionized water, adding reactive surfactants and other reactor ingredients to a reactor under nitrogen, heating the reactor to a temperature of about 68 to 72xc2x0 C., and then adding an initiator charge containing, for example, potassium persulfate. The one or more pre-emulsions are added to the reactor, along with any accelerators, such as sodium formaldehyde sulfoxylate (hydroxymethane sulfinic acid, sodium salt). Any post-reaction charges are added, the reactor is cooled to below about 35xc2x0 C., and the emulsion polymer is filtered. Before filtering the reaction mixture, preferably a biocide is added to prevent bacterial growth. An example of a suitable biocide is Kathon LX (available as a 1.5% solution from Rohm and Haas, Philadelphia, Pa.).
Alternatively, sequential polymerization can be used, wherein the monomers are allowed to react in distinct stages. Specifically, separate first and second pre-emulsions of monomers, surfactants, initiators and other components are prepared, a reactor is charged with an initial surfactant solution and catalyst solution, the first pre-emulsion of monomers is gradually fed into the reactor, and polymerization is initiated and allowed to propagate. After polymerization of the first pre-emulsion, the second pre-emulsion is gradually fed into the reactor, and polymerization continues. The result is a copolymer system of emulsified copolymer particles quite distinct from emulsion copolymers prepared by batch polymerization, as described above. Although not bound by theory, it is believed that sequential polymerization of the two monomeric pre-emulsions results in an emulsion of domain-type copolymeric particles, each having an inner core of a first copolymeric composition and an outer shell or region of a second copolymeric composition, partially or totally encapsulating the core.
Polymer molecular weight is an important property of the PSA and should be controlled for optimum performance. Preferred adhesive copolymers according to the present invention have relatively low weight-average molecular weights (Mw), preferably less than about 120,000, more preferably from about 40,000 to about 100,000, even more preferably from about 55,000 to about 90,000. It is preferred that the molecular weight of the polymer be generally less than 120,000 because larger polymers tend to cause grit, decreasing the dissolvability of the polymer. The low molecular weight polymers of the present invention can be obtained using n-dodecyl mercaptan or another suitable chain transfer agent.
In a particularly preferred embodiment of the invention, the resulting PSA copolymer is compounded or blended with one or more post-additives that improve the dissolvability and/or ghosting characteristics of the PSA. Suitable post-additives for use in connection with the invention include surfactants, plasticizers and mineral salts.
It has been found that the addition of one or more surfactants tends to improve the dissolvability of the PSA. Examples of suitable surfactants for use in the present invention include Disponil A-1080 (a nonionic ethoxylate mixture, commercially available from Henkel), Stepan 1861-68 (an ammonium salt of 3 mole phosphate ester, commercially available from Stepan Company), and AR-150 (a nonionic ethoxylated wood rosin surfactant, commercially available from Hercules). The post-additive surfactant (or mixture of surfactants) is preferably added to the copolymer in a total amount ranging from about 0.4% to about 10.0% by weight, more preferably from about 2% to about 6% by weight, based on the total weight of the copolymer.
It has been found that the addition of one or more plasticizers improves both the dissolvability and the ghosting characteristics of the PSA. Water-soluble polymeric plasticizers are preferred to prevent migration or passage of low molecular weight species out of the dried adhesive film and into the label facestock or to the adhesive/substrate interface causing adhesive deadening. Examples of suitable plasticizers for use in connection with the invention include polymeric plasticizers, such as Plasthall 7050 (a monomeric dialkylether diester plasticizer, commercially available from CP Hall, Memphis, Tenn.) and Plasthall P-900 (a polymeric phthalate plasticizer, commercially available from CP Hall). The one or more plasticizers are preferably added to the copolymer in a total amount ranging from about 0.1 to about 3.0% by weight, more preferably from about 0.5% to about 1.5% by weight, based on the total weight of the copolymer.
The addition of one or more mineral salts also improves the dissolvability of the PSA. Suitable mineral salts for use in connection with the invention include potassium acetate, calcium acetate, magnesium acetate, and ammonium acetate. The one or more mineral salts are preferably added to the copolymer in a total amount ranging from about 0.5% to about 15.0% by weight, based on the weight of the copolymer.
After polymerization and the addition of any post-additives, the PSAs of the invention can be used to prepare a label or other adhesive laminate construction using fabrication techniques well known in the art. For example, the PSA can be coated on a release liner (such as a siliconized paper or film), air- or oven-dried, and then laminated to a suitable facestock. Alternatively, the PSA can be coated directly on a facestock, dried, and then protected with a release liner. The PSA is applied at a desirable coat weight (conveniently measured on a dried basis), which generally lies within the range of about 10 to about 50 grams per square meter (g/m2 or xe2x80x9cgsmxe2x80x9d), more preferably within the range of about 15 to about 25 gsm.
Preferred facestocks for use in connection with the invention are dissolvable facestocks, preferably made of paper. As used herein, the term xe2x80x9cdissolvable facestockxe2x80x9d refers to a facestock that disintegrates or disperses upon contact with water. An example of such as facestock is DISSOLVO dissolving paper, which is a Japanese wood-free paper (manufactured by Mishima Paper Co., Tokyo, Japan).
Preferably the adhesive, when coated on a dissolvable acetate facestock and subjected to a dissolvability test, dissolves in less than about 40 seconds, more preferably less than about 33 seconds, still more preferably less than about 30 seconds. As used herein, the term xe2x80x9cdissolvability testxe2x80x9d refers to the following test. An adhesive label (1xe2x80x3xc3x971xe2x80x3), is prepared with an adhesive coated on a dissolvable acetate facestock (commercially available from Mitsui) and applied on an insoluble test panel made of LEXAN. The label is dwelled on the test panel for a time period of about fifteen minutes at 25xc2x0 C. After dwelling for the indicated time, the test panel is immersed in a water bath at 60xc2x0 C. and moved back and forth slowly. The time for complete dissolution of the test sample is observed and noted.